Gas turbine engines typically include a combustor in which carbonaceous fuel is combusted with an oxidant, most usually air, to produce hot gases of combustion. Most frequently the hot gases of combustion are thereafter diluted with cooler air and then directed through a turbine nozzle which in turn directs the gases against a turbine wheel to drive the same. Large temperature gradients and high operating temperatures in those parts of turbine engines subjected to the hot gases of combustion have long been known to be undesirable. Large temperature gradients are undesirable because of large internal stresses that are generated when one part of a component operates at one temperature and another part operates at a substantially different temperature due to the differences in thermal expansion. The high temperature gas may require the use of exotic materials in constructing turbine components in order to withstand fatigue, and the use of such materials substantially increases the cost of building a turbine.
Consequently it is customary to inject so called "dilution air" into the gases of combustion prior to their application to the turbine wheel and the turbine nozzle which directs the gases thereat. Typically, it is desired to achieve a uniform circumferential mixing of the dilution air with the gases of combustion which produces a specific shape of radial temperature profile at the turbine wheel inlet which is usually not flat. In an optimal case there will be a complete mixing of the dilution air with the gases of combustion such that a uniform temperature of a stream of combined gases of combustion and dilution air is achieved. When and if such a state can occur, the operating temperature of the component can be adequately regulated by controlling, through suitable design parameters, the amount of dilution air in proportion to the gases of combustion. At the same time, severe temperature gradients will be nonexistent because all parts of the gas stream being applied to the turbine nozzle and thus to the turbine wheel will be at substantially equal temperatures.
Perfect circumferential mixing cannot be obtained in practice although it may be approached in large size turbines. This follows because the size of the components is such that there is substantial residence time of combustion gases and dilution air in a combustor or the like prior to the application to a turbine nozzle so as to allow fairly thorough mixing. However, in other cases the residence time is extremely short and adequate mixing will not necessarily occur without undesirably increasing the size of the components.
In order to aid in the attainment of desired temperature gradients within the combustor, dilution air can be injected into the combustion chamber. This can be done so as to produce a localized cooling air film on the inwardly facing surfaces or walls of the combustion chamber. As will be appreciated, these dilution air holes help to maintain as close as possible uniform and acceptable temperature gradients within combustion chambers.
Another problem is intertwined with the problem already discussed hereinabove. During the combustion process there is a tendency for carbon build-up to occur as a result of incomplete combustion. Such is undesirable from the standpoint that incomplete combustion reduces the efficiency of operation of the turbine. From another viewpoint, this is even more undesirable since pieces of carbon may break off and be swept throughout the engine. Such carbon can cause erosion of engine parts and reduce the life of the engine.
The carbon build-up can be especially acute at or near the parts of the combustion chamber in which dilution air is introduced since particularly in this area the temperature will unavoidably be less than elsewhere in the combustion chamber. Insufficient vaporization of the hydrocarbon fuel often results due to relatively low combustor wall temperatures, low vapor pressure of heavy liquid fuels used and low residence time within the combustor. As turbine engines use heavier molecular weight fuels the problem of elemental carbon formation becomes even more pronounced. These factors frequently result in liquid fuel droplets impacting and attaching to the relatively cool combustor wall and once the droplets attach to the combustor wall, combustion reactions will not proceed to completion but, instead, decomposition of the hydrocarbon molecules will leave carbon at the wall which may grow to become very large deposits. While one undesirable aspect of the carbon build-up is that it may interfere with heat transfer, the more serious problem is that the carbon may break free and damage engine parts.
In air blast fuel injectors, the air passageways are susceptible to being plugged by carbon lumps from carbon which has formed on internal combustor walls and which fall out of a combustor through a dilution air hole. This may typically occur when the engine is shut down, or as the engine is in the process of being shut down, whereupon in a subsequent restart, the carbon can be carried forward by the incoming air to lodge in a fuel injector's air passageway. Obviously if the air blast fuel injector air passageway is blocked, poor fuel atomization will occur and, consequently poor engine performance will result.
The present invention is directed in overcoming one or more of these problems.